The environmental impact of Aviation - Part 2

Exploring the technical solutions that could help make flying less bad for the planet

Welcome back to the second part of this exploration into the impacts of aviation on the environment (here’s part 1 if you missed it). I meant to wrap this up sooner, but these things always take longer than expected! This part dives into the fun stuff - we’ll be exploring some of the potential technical solutions that could make their way onto future aircraft, as well as why we will or won’t see them any time soon. Apologies for the lack of images - the text alone is right on the limit of what GMail will show without clipping, and I didn’t feel it was fair to split it again!

A technical interlude

Firstly, we must take a brief but relevant segue into the mechanics of flight. Flying is dominated by weight - perhaps understandably, given the importance of gravity in our lives, overcoming it takes a lot of energy. This drives how aircraft are designed, and how they have evolved - the need for light weight and huge power.

Whilst small aircraft tend to use piston engines similar to those found on a car, most large aircraft use some variation on jet turbines to propel themselves through the air. Invented in the 30s, jet turbines offered a huge boost in power for the same weight as a piston engine, and were one of the key inventions that made the airliners we fly on today possible. However, that power comes with a hefty fuel cost. Today, most commercial aircraft use either turboprop or turbofan engines - basically jet turbines driving a propeller or a large fan respectively. These improve fuel efficiency significantly, but planes today still need enormous amounts of fuel to fly. Jet airliners generally run on specific aviation fuel that is largely kerosene based - not too far from the gasoline or diesel fuels most cars and trucks run on. However, they burn vast quantities of the stuff - airliners usually carry anywhere from 20-45% of their max take-off weight in fuel for each flight. An Airbus A380 can carry over 200 tons of fuel for a single flight!

These requirements of light weight and high power combined with strict safety standards severely limits the potential solutions. However, there are a surprising number of technologies that will reduce or eliminate aircraft emissions, although each comes with their own collection of drawbacks.

The Solutions

Fuels

First, let’s look at where our future aircraft gets its power from. Currently, due to the high power and strict weight requirements, aviation-grade kerosene makes up the overwhelming majority of fuel used, but there are a few potential alternatives, both now and looking forward into the future. Let’s look at the three main contenders.

SAFs

The most straight-forward solution is one we touched upon in part 1 - SAFs, or Sustainable Aviation Fuels. These are fuels that aim to reduce the climate impact of flying whilst being drop-in replacements for fossil-based fuels used today. These broadly fall into two categories - biofuels and synfuels. 

Biofuels are fuels made from biological materials - plants or other biological matter. Not all biofuels are renewable, but those that are aim to sequester carbon within the plant matter which can then be burned as fuel, thus creating (in theory) a carbon neutral cycle - plants absorb carbon, planes burn plant-based fuel and emit the carbon. However, in practice, it gets a lot more tangled than that. Biofuels are a messy and complex topic, as the plants they are made from must compete for space with countless alternatives - for example food crops or forest which was already sequestering carbon. Biofuels are also still conventional fuels, and burning them has all of the climate impacts of fossil fuels - CO2, NOx, contrails.

Synfuels, or synthetic fuels, are a similar concept. Here, the idea is to create an artificial fuel from base chemicals (carbon monoxide and hydrogen) rather than from plants. For renewable synfuels, that usually means using CO2 captured from the air or from carbon capture systems in power plants, along with green hydrogen created using electrolysis with renewable energy. An advantage of synfuels is that the exact chemical composition of the resulting fuel can be precisely controlled, helping both performance and emissions over fossil or bio fuels. For example, synfuels can be engineered with fewer aromatic compounds reducing soot during combustion, reducing NOx emissions and contrail production. Depending upon the source of the carbon, the power used to create the fuels and their chemical makeup, the overall climate impact may be 30-60% less than current fuels.

That improved climate impact does come at a significant energy cost. 1kWh equivalent of synfuel takes between 2.8-4.6kWh of input energy to create, which would massively increase the energy footprint of aviation should the fuel become widespread.

Overall, SAFs make an excellent bridge solution given their drop-in nature, but they are unlikely to reduce environmental impact more than around 60%. Regulators are starting to permit fuel mixes containing SAFs on a number of routes, but thus far take-up has been low. SAFs are still significantly more expensive than conventional fuels, and the volume available is tiny. However, as more and more airlines start to consider their environmental footprint, companies are starting to commit to large orders of SAFs, which in turn spurs more production, and with scale, prices will fall.

Hydrogen

Another option on the roster of alternative fuels is hydrogen. Hydrogen’s major ace card is that it is a truly clean fuel - when burned with oxygen, it produces water as the only byproduct (when burned in air, there will be some NOx produced from the nitrogen present). Hydrogen has been touted as the clean fuel of the future for decades, with hydrogen cars, trucks, boats, trains and more proposed over the years. So what benefits does it have, and why has it failed to take off (pun intended) so far?

The clean-burning nature of hydrogen makes it a prime candidate for reducing emissions in any situation involving fossil fuels, but it has a few other tricks up its sleeve. Firstly, it can be generated by splitting water using electricity (a form of electrolysis), allowing it to be generated easily from clean energy sources. Second, it can be used in many conventional fossil fuel-burning engines, from those in cars to jet turbines, removing the need for a new propulsion method. The ace card is the hydrogen fuel cell - a device that essentially reverses electrolysis, combining hydrogen and oxygen to produce electricity at a high efficiency. All this makes hydrogen clean and very versatile - it can be used in most conventional engines as well as a power source for electric propulsion.

However, there’s a ‘but’ - all these features do not come without downsides. Hydrogen is the lightest element in the periodic table, and that brings some unique quirks. The molecules are so small that they leak out of many pipes and tanks, and can cause metals to become brittle. It also has a very low density, meaning that gaseous hydrogen must be stored at very high pressures to avoid it taking up vast volumes. Liquid hydrogen is more dense, though even then it still has a third of the volumetric energy of kerosene (though 3-4x the energy per unit weight!). However, hydrogen boils at a chilly −252.87°C, which means that storing liquid hydrogen runs into a host of cryogenic complexities.

Then there’s generation of hydrogen. Currently, the vast majority of hydrogen is generated from steam cracking of natural gas, and is therefore not emissions-free. Green hydrogen is possible, but requires large amounts of clean energy - something that is in relatively short supply currently. As such, prices for green hydrogen are 4-7x those of kerosene currently, though the EU is one of a number of regions making a big push to lower prices by ramping up production in the next decade.

Hydrogen in cars and ground transport has faced an uphill struggle - the fuel is currently expensive, difficult to store, and in short supply. Fuel cells are also still a relatively undeveloped technology, and as such are only around 70-80% efficient, and suffer from poor power to weight ratios. When weighed up against batteries and electric motors, which are efficient, simple and can hook into the existing power grid, there was no contest.

However, in aircraft, the conditions are significantly different. Aircraft travel huge distances, and require an energy source that is as light as possible. Cost is less of a factor, and as such hydrogen looks like one of the most promising routes to almost completely decarbonise aviation. Switching from conventional fuel to hydrogen (burnt in jet turbines) would reduce overall climate impact by 50-75%, and using fuel cells could push that to 90-95%.

There are still major barriers however. Fuel cells need to become much lighter and more powerful, and the insulated tanks required for liquid hydrogen also must become much lighter. There is the not insignificant burden of regulation and certification to overcome, not to mention building out a hydrogen supply at every major airport worldwide. Nevertheless, it’s enough of a serious contender that Airbus is making a multi-billion dollar push to develop hydrogen aircraft, calling the fuel ‘the most important transition this industry has ever seen’.

Full Electric

The final energy source to consider is, of course, batteries. Batteries have come on a huge way in the last two decades, steadily increasing in energy density and dropping vastly in price. However, compared to kerosene, batteries have 40-50x less energy per unit weight  - a gulf that will not be bridged by anything other than a revolutionary new battery technology in the next couple of decades.

Batteries also have another weight disadvantage to overcome - unlike fuel, they aren’t consumed during the journey. The huge amounts of fuel required by conventional aircraft means that by the end of a flight, they are significantly lighter, and thus require less energy to propel through the air. Batteries are sadly just as heavy when fully charged as when they’re flat as a pancake.

Does that mean we won’t ever see battery-electric aircraft? Not at all, but they will dominate in smaller, lighter aircraft that cover short distances - realms where the advantages of electric propulsion outweigh the disadvantages of heavy batteries. A number of companies are looking to introduce electric versions of pilot trainer aircraft - small, lightweight two seat aircraft that fly for an hour or so at a time. Operators of short distance propeller aircraft have also prototyped conversions of small passenger planes, such as the de Havilland Beaver and Cessna Caravan, which can carry 10 or so passengers a few hundred miles at most.

The other realm seeing huge attention at the moment is the eVTOL air taxi market, often referred to as Urban Air Mobility (UAM). There are hundreds of companies trying their hand at some form of small, vertical take-off short range aircraft, though only a few have actually flown anything. This regime fits electric flight well - the distances are small, and electric motors are as well suited as any to the power requirements of vertical propulsion. However, the whole market is new, meaning it faces vast regulatory and certification hurdles, not to mention pricing challenges. It also does next to nothing to decarbonise existing aviation, seeking instead to move commuters from cars on the ground to cars in the air. In a very Silicon Valley move, it seeks to solve congestion with a vastly complex, over-engineered solution when better public transit and improved city planning would likely solve the core issue far more effectively.

Propulsion

The next area of aircraft to look at is propulsion. Modern aircraft largely either use turboprop or turbofan engines - essentially these are jet turbines with either a propeller or a big fan on the front respectively. Each works best in different domains, which determines which aircraft use what. Turboprops have high propulsive efficiency, but become rapidly less efficient above around 0.6-0.7 Mach. Turbofans are only efficient at much higher speeds - 0.8 Mach and above, hence turboprops dominate on slower, shorter distance aircraft, whilst most airliners use turbofans for their improved performance at higher speed.

It used to be common for longer range jet aircraft to have 3 or 4 engines, principally for redundancy. There are no lay-bys in the sky, and especially on long oceanic flights, the aircraft must be able to keep flying long enough to reach safety should an engine failure occur. Multiple engines add a safety margin, but introduce significant cost and maintenance overhead - jet turbines are complex machines, and more engines means higher running expenses.

However, with time twin engined airliners have become reliable enough that rules governing how far they could fly from a diversion airport (known as ETOPS) have been increased significantly. Modern engines are reliable enough that the recent Airbus A350 received an ETOPS-370 rating, meaning it can fly for 370 minutes on only a single engine. This means even very large aircraft can use only two engines without sacrificing range or route coverage. Larger jet turbines are generally more efficient, giving another boost to twin-jets. But what does the future hold for propulsion?

Hybrid Aircraft

One possibility is to combine electric motors with jet turbines for still further improved efficiency, much as in hybrid cars. There are a host of different ways of combining the two modes, each with different benefits and drawbacks.

A parallel hybrid would see motors and engines mechanically linked, with either or both combined able to power the aircraft. In this way, engines could be made smaller, and use the motors as a booster for high-power periods of flight such as take-off. A bonus is that the motor could work in reverse as a generator for in-flight power.

An alternative is the series hybrid. Here, the aircraft is propelled by electric motors, but the electricity is provided by a jet turbine running a generator. This decouples the propulsion from the turbine, which creates more flexibility in design, but it relies on each stage of the powertrain being efficient - the benefits of electric propulsion must outweigh converting engine power to electric and back to motive force again.

Hybrids are generally assumed to have a battery, but given their high weight, an alternative is to avoid batteries altogether - an architecture known as turbo-electric. This is a well proven approach in trains and ships, where it avoids the complexity of heavy mechanical gearboxes, but whether it can translate effectively to aircraft remains to be seen.

Electric propulsion

But why would you want a partially or fully electric propulsion system? Pure electric propulsion is highly energy efficient versus combustion engines, but if an engine is involved, then there must be other benefits to justify the conversion from jet turbine to electric motors.

Engines are large, heavy, and require lots of maintenance. This in turn limits where they can be placed on an aircraft. Electric motors, however, come in all shapes and sizes, are mechanically simple, and only require a power cable. This means they can be located all across the airframe to reduce drag or increase power.

This can take a whole variety of forms. Wingtip motors (see the Eviation Alice) can reduce wingtip vortex drag. A tail motor can re-energise the boundary layer of air slowed down by the fuselage, reducing skin drag. Motors can be used for blown flaps, increasing take-off performance and reducing mechanical complexity. Scattering motors across the wing can increase lift, reducing the wing area required. As a handy side effect, having more motors gives redundancy, which means less power overhead. A downside of modern aircraft having just two engines is that they need to fly safely should an engine fail, meaning that each engine provides a lot more power than is needed. Electric motors can reduce that overhead by having a high level of redundancy, without the maintenance and efficiency disadvantages that come with many jet turbines.

There are a few hurdles to cross before we see large electric or hybrid aircraft however. Weight and power, as always, are critical. A typical wide-body aircraft may need as much as 60MW of propulsive power, and creating motors with that power at a similar weight to jet turbines is a significant challenge. Similarly, other pieces in the electric power train such as inverters are also not light nor powerful enough yet, though rapid progress is being made on both fronts. A further challenge is making light-weight cables that can safely transmit incredibly high voltages and/or currents without introducing a fire hazard.

Boundary layer ingestion

Another engine-related efficiency measure proposed is boundary layer ingestion. The boundary layer is the layer of air immediately next to a surface such as an aircraft’s skin. This layer is slowed by viscous effects, and adds a dragging force to the aircraft. Boundary Layer Ingestion places propulsors to suck up this boundary layer air, speeding it up and negating much of the drag. This technique has been proposed in a number of efficient concept aircraft such as the Aurora D8 and the NASA STARC-ABL, but not yet on any production aircraft. Despite the efficiency gains, the idea faces difficulties with managing the stresses inherent in running fans in slowed or turbulent air.

Ultra-high bypass engines

A turbofan engine uses a big fan at the front of the engine to pull air both through and around the engine. The ratio of air going through the turbine in the middle versus around the outside is known as the bypass ratio, and generally, a higher bypass ratio (i.e. more air going around rather than through) provides lower fuel consumption for the same thrust (less air is being combusted to move the same overall volume of air). This drive has seen turbofan engines improve from bypass ratios of 0.5-1:1 up to 10-12:1 on the latest jetliners. This trend is likely to continue, with major engine manufacturers chasing 15:1 for their latest designs.

Another option being considered is the propfan - a hybrid of a turboprop and turbofan which in theory captures the fuel efficiency of a turbofan but with the speed of a turbofan. First proposed in the depths of the oil crisis in the 1970s, a few designs were developed in the ‘80s, but as oil prices dropped, development largely stalled. The designs looked promising, offering considerable fuel efficiency gains, but had major issues with noise that will need solving before they see wider acceptance.

More Electric Aircraft

Current aircraft have a plethora of different systems all running in parallel to ensure everything runs smoothly. In addition to the engines and their fuel system, there’s also an electrical system, for running everything from lights to computers and more, pneumatics for brakes and landing gear doors, hydraulics for flight surfaces and landing gear, and more. Given the strict safety requirements, these systems are often replicated for redundancy, adding a lot of weight and complexity.

Significant efficiency savings can be had from moving most if not all of these systems to electric equivalents, simplifying the architecture of the aircraft significantly. In addition, electric equivalents to hydraulic or pneumatic systems are often lighter, allowing the plane to consume less fuel through weight reductions. These could either be run from the engines or via a dedicated fuel cell or similar electrical generation system.

This was one of the major innovations in the Boeing 787. Whilst not every system is electric, much more of the aircraft is powered this way than its predecessors, giving an estimated 3% fuel savings. However, even this partial shift was not simple - the aircraft was plagued by battery issues, which led to the plane being grounded after a series of fires. Nevertheless, now that the teething issues have mostly been worked out, this looks like a likely option for many new aircraft, although it’s not generally considered a cost-effective option to retrofit old aircraft.

Other future technologies

Blended surfaces and laminar flow

Smooth surfaces are crucial for reducing drag on aircraft flying at high speeds. Modern aircraft still have a number of abrupt edges, from wingtips to flight control surfaces, all of which can induce drag. The gains from smoothing an individual part may be small, but added up across an entire airframe, this can yield a few percent of additional efficiency. NASA has performed testing on a business jet fitted with ‘seamless’ flaps that eliminate the gaps created by conventional moving flight surfaces.

Another area that has seen much research is laminar flow. To oversimplify massively (aero experts, I’m sorry!), moving air largely falls into two regimes - laminar flow, which is smooth and stable, and turbulent flow, which is chaotic and unstable. Turbulence around an object moving through air will cause drag, increasing the energy required to propel it forwards. For aircraft, therefore, it is desirable to keep flow laminar around the vehicle for as long as possible. There have been many efforts to improve the levels of laminar flow around wings and other surfaces, both through passive design features and more active approaches. 

Composites

Aircraft have used aluminium alloys as their material of choice since the 1920s, and whilst there have been significant advances in airframe design since then, the materials have remained largely the same. However, in recent years, composite materials have progressed in leaps and bounds, offering greater strength and lighter weight. They are more complex to design and work with, and they behave very differently to metals, with vastly different failure modes. This complexity combined with conservative safety regulations slowed their adoption, but recent aircraft programs such as the Boeing 787 and Airbus A350 have embraced composites to a large degree. Future aircraft will likely see a further development of this trend, with composites offering simpler, lighter airframes.

Novel aircraft designs

One of the most interesting areas for me as an aviation enthusiast is new, innovative aircraft designs. After some interesting experimental flourishes in the early decades of flight, aircraft have largely not changed in 70 or more years, sticking to a tried-and-tested approach of tube-and-wings. There are good reasons for this design - a tube efficiently carries the load from the pressurisation needed to carry passengers safely at high altitude. It has a narrow frontal area, reducing drag, and is also scalable, allowing manufacturers to resize an aircraft easily to cater to different markets. The wing, engine and fuselage are also aerodynamically fairly distinct, allowing them to be optimised without interfering with one another. However, researchers in search of more efficiency have come up with many alternative ideas, a few of which we’ll take a brief look at here.

The ‘double bubble’

This concept is not too dissimilar to current aircraft, but widens the fuselage significantly by joining two tubes together - hence ‘double bubble’. This allows the body to add some lift, meaning the wings can be smaller, and also allows for boundary layer ingestion, further improving efficiency. This design was proposed for a NASA next generation aircraft competition some years back, but unfortunately has seen little progress since.

Laminar flow aircraft

We’ve already covered the benefits of improving laminar flow over surfaces, but what if you designed an entire aircraft to optimise for this? In that case, you end up with an unusual-looking, but incredibly efficient aircraft like the Otto Aviation Celera, which performs like a business jet, but cuts fuel consumption massively. It is unfortunately limited to relatively small aircraft - the design can only be scaled up a small amount before the benefits fade away - so it’s unlikely we’ll see such designs for airliners any time soon.

Blended wing bodies

Whilst most efficient aircraft technologies assume a similar design to current aircraft, with a roughly tubular fuselage and separate wings, some researchers have proposed more radical designs. One of these is the blended wing body (BWB), sometimes known as a hybrid wing body. This is a design where fuselage and wings are smoothly blended together, somewhere between a flying wing and a conventional aircraft.

This design reduces the ‘wetted area’ of the aircraft - the surface area exposed to airflow - and thus drag. Some designs also give the fuselage an airfoil shape, allowing the entire aircraft to generate lift. Different studies have shown varying levels of improvement, but the design could yield 20-30% or more efficiency improvements in cruise. Most variants also place the engines on top, which also cuts noise massively over current aircraft. BWB designs also integrate well with hybrid and distributed propulsion designs, as these can utilise boundary layer ingestion, offering further efficiency gains and noise reductions.

Whilst a huge amount of research effort has flowed into the designs from NASA, Boeing and others, nothing more than scale aircraft have flown so far. As the design is so different, there are a host of technical challenges to overcome, from pressurising a cabin that is not round, flight stability, cabin layout, and more. Blended wing body designs offer some of the biggest potential gains, but with so little in common with conventional designs, also some of the highest development risks.

Future aircraft

So given all these technologies (and more I didn’t mention), what does the future of aircraft look like? Some of the technologies covered are fairly certain - ultra high-bypass engines, composite airframes and more electric technologies are already starting to be seen on the newest airliners - but beyond that the picture becomes less certain. The heavy burden of regulation, and the domination of the industry by a couple of enormous incumbents means that more radical solutions such as hybrid-electric aircraft or novel airframe designs are unlikely to come from within.

However, the barriers to entry for a new entrant are huge. The costs to develop a new aircraft from the ground up are astronomical, and the complexity is similarly huge. Nevertheless, I think disruption is needed if the industry is to radically change. Similar to Tesla in the car industry, a radical new player would likely force the industry giants to innovate or lose market share, moving the field forward even if the new company ultimately did not succeed.

I remain hopeful that in at least some markets, other forms of transport such as high-speed rail could supplant much of the need for shorter flights, but longer distance journeys will be hard to replace. Many airlines will start using SAFs, particularly in areas where carbon taxes on aviation are implemented. Electric and hybrid flight seems likely, but will be a decade away at least, even for short haul applications. Long haul flights may never be fully electrified, but new aircraft designs, fuels and turbo-electric propulsion could massively reduce the impact of these journeys, although there too such changes may take 10-20 years to realise.

Fin

If you made it all the way through, well done, and thank you for persisting! This is my first major deep dive for Forge the Future, and I’d love to know what you thought, either by leaving a comment below, or via email. There’s a whole host of other topics I’d love to explore in similar depth, from the ins and outs of batteries, to deforestation, high-speed rail, and much more. I’m also always open to suggestions - if there’s a particular topic you’d like to know more about, please let me know!